Field of the Invention
The present invention relates to an image capturing apparatus and a method for controlling the same, and, in particular, relates to an exposure control technique.
Description of the Related Art
There is known to be a method for suppressing blown-out highlights (overexposure) and blocked-up shadows (underexposure) in a captured image by shooting with an exposure that is lower than the standard exposure (under exposure) and adjusting the luminance by performing gamma correction (tone correction), and expanding the dynamic range compared to the dynamic range at the time of the standard exposure. Gamma correction characteristics (also referred to as a gamma curve) are characteristics that define the correspondence between an input level range based on a set dynamic range and a predetermined output level range, and it is possible to control how to reflect the tone property of the input level on the output level by changing the gamma correction characteristics (the shape of the gamma curve).
Some methods for expanding the dynamic range and changing gamma correction characteristics have been suggested. Japanese Patent Laid-Open No. 2004-120511 suggests a method for effectively using the tone of a high luminance portion as well by expanding the dynamic range and smoothing the inclination of the high luminance portion of the gamma correction characteristics. According to this method, even in a state where the dynamic range is narrow, the tone of the high luminance portion can be effectively used by changing the inclination of the high luminance portion of the tone characteristics.
In addition, Japanese Patent Laid-Open No. 2006-81037 suggests a method for making the incident light amount (maximum reflectance of an object) that is to correspond to the maximum output of an image sensor variable, and changing the gamma correction characteristics so as to maintain the output value that corresponds to a reference reflectance, even in the case where the maximum reflectance is changed. In this method, in the case of increasing the maximum reflectance, the gamma value is brought close to 0, and in the case of decreasing the maximum reflectance, the gamma value is brought close to 1. Accordingly, if the maximum reflectance is high, the contrast is reduced, and if the maximum reflectance is low, the contrast is increased.
In the case of expanding the dynamic range by combining shooting with an underexposure and changing the gamma correction characteristics, gamma correction compensates for the insufficiency in exposure, and thus the gamma correction involves signal amplification, causing a decrease in the S/N ratio. Therefore, from the viewpoint of the S/N ratio, it is preferred not to perform dynamic range expansion more than necessary. Therefore, in the case where blown-out highlights occur in an image under a certain shooting condition, it is thought that a user will desire the least amount of dynamic range expansion that can reduce the blown-out highlights. However, in the portion with blown-out highlights, the photometric values are also saturated, and it is not possible to specify the level of the incident light in the portion with blown-out highlights. Therefore, there is a problem in that the user cannot figure out a dynamic range expansion amount necessary to avoid the blown-out highlights.
In order to solve this problem, a method for temporarily expanding the dynamic range and identifying the highest level of incident light, and then setting a shooting condition for performing the least amount of dynamic range expansion that is necessary is conceivable. However, if the operation for specifying the highest level of incident light takes time, photo opportunities are missed, and usability deteriorates.
However, it is necessary to change the exposure to an underexposure in order to change the dynamic range, as described above, and a certain period of time is necessary for the change of the exposure. Usually, methods for changing the exposure include methods for changing the aperture value, inserting/removing an ND filter, changing the electronic shutter speed, changing the ISO sensitivity and the like. Among them, changing the electronic shutter speed and changing the ISO sensitivity correspond to changing the semiconductor device settings of circuits in the image sensor or the signal processing circuit, and thus take a short time. On the other hand, changing the aperture value and insertion/removal of the ND filter are operations performed via a mechanical mechanism, and thus a long period of time to some degree is required from the start of changing the exposure to the end.
For example, when operating the diaphragm using a stepping motor, consider the operation speed in the case where the aperture changes by ⅛ per pulse and the motor rotation speed is 500 pps (pulses per second) is considered. In the case of changing the exposure by one stage, the stepping motor needs to be operated by 8 pulses. Therefore, the period of time when the diaphragm is operating is:8 pulses÷500 pps=0.016 s=16 ms.
In actuality, in order to correctly operate the stepping motor, it is necessary to perform acceleration control for gradually increasing the speed, instead of driving at 500 pps from initial movement. Moreover, the time required for the acceleration is approximately several tens of ms. Also, when stopping the stepping motor, in order to stop at a precise position, it is necessary to perform deceleration control of approximately several tens of ms similarly to the acceleration control.
Regarding insertion/removal of the ND filter as well, an operation via a mechanical mechanism using a stepping motor or the like is common. In particular, with an ND filter incorporated in a video camera for business use, performance that is demanded is high, and thus it is necessary to cover the entire opening diameter. Furthermore, a plurality of ND filters are required so as to be able to be applied to various scenes. Therefore, in the case of operating the ND filter, the ND filter will be operated over a distance longer than the distance over which the opening diameter of the diaphragm is operated, and the operation time becomes longer. Therefore, in the case of expanding the dynamic range, a time lag occurs due to the time required for changing the exposure, causing photo opportunities to be missed. Moreover, there is a problem in that a setting error is caused, or the user is caused to have an unnatural feeling during the use.